Storage of an image in an integrated circuit

ABSTRACT

An integrated circuit including a substrate of a semiconductor material and first metal portions of a first metallization level or of a first via level defining pixels of an image. The pixels are distributed in first pixels, for each of which the first metal portion is connected to the substrate, and in second pixels, for each of which the first metal portion is separated from the substrate by at least one insulating portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. patent application Ser. No. 12/538,336, filed on Aug. 10, 2009, which application is claims priority benefit of French patent application number 08/55628, filed on Aug. 19, 2008, entitled “STORAGE OF AN IMAGE IN AN INTEGRATED CIRCUIT,” which are hereby incorporated by reference to the maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure generally relates to integrated circuits and, more specifically, to the non-volatile storage of an image in an integrated circuit during the circuit manufacturing.

2. Discussion of the Related Art

In many cases, it is needed to permanently store an image in an integrated circuit on manufacturing of the integrated circuit. The image for example shows an identifier of the wafer on which the integrated circuit is manufactured, the position of the integrated circuit on the wafer, an identifier of the wafer batch to which the wafer of the integrated circuit belongs, etc.

Generally, to permanently store an image in the form of binary data in an integrated circuit, a ROM-type memory, an electrically erasable programmable read-only memory (or EEPROM), etc. is generally used.

A disadvantage is that the image to be stored may be unavailable at the time when the etch masks used during the integrated circuit manufacturing are defined. Further, the image to be stored may vary from one wafer to the other, especially when it shows identifiers of wafers and of wafer batches. Since the storage of data in a ROM generally requires the use of at least one specific mask for the forming of the memory cell connections, it is not compatible, for a manufacturing of circuits at an industrial scale, to have to multiply the number of masks to be used.

Further, it is desirable for the stored image to remain accessible even when the integrated circuit is disabled. As an example, when the stored image represents the batch identifier, the wafer identifier, the integrated circuit position, etc., it is generally desired to access this image when the circuit is malfunctioning, to ease the determination of the causes of this malfunction. When the image is stored in a ROM, an EEPROM, etc., it is no longer possible to read it when the circuit is no longer functional.

An example of a method for permanently storing an image at the level of an integrated circuit, enabling the stored image to remain accessible when the circuit is disabled and enabling to easily store different images from one integrated circuit to the other, comprises scanning the upper surface of the integrated circuit with a laser to etch the image in the form of patterns on a portion of the integrated circuit. However, such a method needs to be implemented separately on each integrated circuit and is thus not compatible with the manufacturing of integrated circuits at an industrial scale. Further, the images to be stored may be unavailable at the time when the laser etch method is to be implemented.

SUMMARY OF THE INVENTION

A method for permanently storing an image at the level of an integrated circuit is disclosed, the image remaining accessible when the integrated circuit is disabled, where the image storage can be performed simply and at a decreased cost an industrial scale.

An object of an embodiment of the invention is to enable the image storage at the level of the integrated circuit after the manufacturing of the integrated circuit and possibly after steps of test and/or of assembly of the integrated circuit.

Another object of an embodiment of the invention is to make the reading of the image stored at the level of the integrated circuit particularly simple.

Embodiments of present invention also aim at an integrated circuit at the level of which an image is permanently stored, the image remaining accessible when the integrated circuit is disabled, where the image can be read in a simple way.

To achieve all or part of these objects as well as others, an embodiment of the present invention provides an integrated circuit comprising a substrate of a semiconductor material, first and second pixels, each pixel being formed of a first metal portion of a first metallization or via level, the first metal portion of each first pixel being connected to the substrate and the first metal portion of each second pixel being separated from the substrate by at least one insulating portion, and for each pixel, a capacitive coupling element connecting the pixel to a selection element.

According to an embodiment of the present invention, each pixel is connected, by an electric path, to a second metal portion of a second metallization level closest to the substrate than the first level, and, for each of the first pixels, the associated second metal portion is connected to the substrate, and, for each of the second pixels, the associated second metal portion is separated from the substrate by at least one insulating portion.

According to an embodiment of the present invention, each pixel has a corresponding two-state luminance and/or chrominance attribute, and for each first pixel, the corresponding attribute is at a first state and, for each second pixel, the corresponding attribute is at a second state.

According to an embodiment of the present invention, for each pixel, the associated capacitive coupling element connects the associated second portion to a selection element.

According to an embodiment of the present invention, the capacitive coupling element is a MOS transistor comprising first and second gates, the second gate being floating and being connected to the associated second portion, the first gate being connected to the selection element.

According to an embodiment of the present invention, the capacitive coupling element is a MOS capacitor comprising first and second armatures, the first armature being connected to the associated second portion and the second armature being connected to the selection element.

According to an embodiment of the present invention, the second metal portion associated with each first pixel is connected to the gate of a first MOS transistor having a gate insulator which has not been altered and the second metal portion associated with each second pixel is connected to the gate of a second MOS transistor having a gate insulator which has been altered to be conductive.

According to an embodiment of the present invention, the selection element is a first conduction terminal of a MOS power transistor having a second conduction terminal connected to a power supply terminal and having a gate adapted for receiving a selection signal.

An example of the present invention provides a method for storing an image in an integrated circuit comprising a substrate of a semiconductor material, pixels being formed of first metal portions of a first metallization or via level, each pixel being separated from the substrate by at least one insulating portion, and each pixel being coupled to a selection element via a capacitive coupling element, and a voltage is applied between at least several pixels and the substrate, via the corresponding capacitive coupling element, to alter said insulating portion and electrically connect the first metal portion to the substrate.

According to an embodiment of the present invention, each pixel is connected by an electric path to a second metal portion of a second metallization level, closest to the substrate than the first level, each second metal portion being separated from the substrate by said at least one insulating portion, and, for said at least several pixels, said voltage is applied, via the associated capacitive coupling element, between the second metal portion associated with each of said several pixels and the substrate.

The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified top view of an integrated circuit according to an embodiment of the present invention;

FIG. 2 is a cross-section view of the integrated circuit of FIG. 1 along line A-A;

FIG. 3 is an image obtained in an operation of reading of the image stored at the level of the circuit of FIG. 1;

FIGS. 4A to 4D show the structures obtained at successive steps of a method for manufacturing the circuit of FIG. 2 according to an embodiment of the present invention;

FIG. 5 shows an electric diagram of an integrated circuit according to another embodiment of the present invention;

FIG. 6 is a partial simplified cross-section view of the integrated circuit of FIG. 5;

FIGS. 7A to 7F show the structures obtained at successive steps of a method for manufacturing the circuit of FIG. 6 according to an embodiment of the present invention;

FIG. 8 shows an electric diagram of an integrated circuit according to another embodiment of the present invention;

FIG. 9 is a partial simplified cross-section view of the circuit of FIG. 8;

FIG. 10 shows an electric diagram of an integrated circuit according to another embodiment of the present invention; and

FIG. 11 is a partial simplified cross-section view of the circuit of FIG. 10.

DETAILED DESCRIPTION

For clarity, only those steps and elements which are useful to the understanding of the present invention have been shown and will be described. Further, the same elements have been designated with the same reference numerals in the different drawings and, moreover, as usual in the representation of integrated circuits, the various drawings are not to scale.

In the following description, the assembly of metal tracks simultaneously formed during the integrated circuit manufacturing process is called a metallization level. These may be metal tracks lying at the surface of an insulating layer or leveling the surface of an insulating layer. A conductive portion crossing an insulating layer and connecting a metal track of a given metallization level to a metal track of an adjacent metallization level is called a via. The assembly of vias simultaneously formed during the integrated circuit manufacturing process is called a via level. Vias may be formed simultaneously to the metal tracks of a given metallization level. In this case, the via and metallization levels are confounded.

The present disclosure relates to storing an image at the level of a portion of an integrated circuit by means of pixels formed by metal portions of a same metallization level or of a same via level, preferably the last or the penultimate metallization level or the intermediary via level between the last and penultimate metallization levels. Each pixel has an associated chrominance and/or luminance attribute that can take two states. The image is stored by letting the pixels float or by electrically connecting them to the substrate during a write operation. The image reading may be performed by adapted imaging techniques enabling to make out floating pixels from connected pixels. Such techniques for example may be passive voltage contrast imaging techniques.

FIG. 1 is a partial simplified top view of an integrated circuit 10 according to an embodiment of the present invention and FIG. 2 shows a cross-section of circuit 10 of FIG. 1 along line A-A.

Circuit 10 comprises a substrate 12 of an undoped or P-type doped semiconductor material, for example, polysilicon, having an upper surface 13. Substrate 12 comprises active areas, for example, N-type doped areas, which extend into substrate 12 from upper surface 13 and which have an elongated shape. As an example, four parallel active areas C₁ to C₄ are shown in FIG. 1 in stripe-dot lines and two adjacent active areas C₁ and C₂ are shown in FIG. 1. Active areas C₁ to C₄ are separated by insulating regions 16. An insulating layer 18 covers surface 13. Insulating layer 18 is covered with a stack of insulating layers INS₁ to ISN₄.

In the present embodiment, integrated circuit 10 comprises metal portions formed in four different metallization levels M, where k is an integer varying from 1 to 4. The metal portions of first metallization level M₁ are the closest to substrate 12 and the metal portions of last metallization level M₄ are the most distant from substrate 12. As an example, the conductive tracks of metallization levels M₁ to M₃ are made of copper and the metal tracks of last metallization level M₄ are made of aluminum. In the present embodiment, the metal portions of metallization level M_(k) are arranged on insulating layer INS_(k).

Pixels Pix_(i,j), where i and j are integers varying between 1 and 4 in the present example, are formed above active areas C₁ to C₄. In FIG. 1, each pixel Pix_(i,j) has been represented by a square in dotted lines. As an example, the pixels are arranged in rows and in columns. Active areas C₁ to C₄ are oriented along the pixel columns, four pixels Pix_(1,j), Pix_(2,j), Pix_(3,j), and Pix_(4,j) being associated with each active area C_(j).

In the present embodiment, each pixel Pix_(i,j) is formed by a metal portion of penultimate level M₃. Each metal portion Pix_(i,j) is connected to a metal portion MP_(i,j) of first metallization level M₁ by an electric path EP_(i,j) formed by the serializing of vias and of a metal portion of metallization level M₂. Each metal portion MP_(i,j) is connected to a via V_(i,j) crossing first insulating layer INS₁ and extending all the way to insulating layer 18. A write area of pixel WZ_(i,j), which corresponds to the portion of insulating layer 18 interposed between via V_(i,j) and active area C_(j), is associated with each pixel Pix_(i,j).

Parallel metal tracks R₁ to R₄ of last metallization level M₄ extend along the pixel rows. Each metal track R_(i) is connected, by vias V′_(i,j) crossing insulating layer INS₃, to pixels Pix_(i,j), where j varies from 1 to 4. In other words, each track R₁ and R₄ is connected to all the pixels in a same row.

The storage of an image is obtained, for each pixel Pix_(i,j), in a write operation performed once integrated circuit 10 has been manufactured. It comprises altering, for some pixels Pix_(i j), write area WZ_(i,j) to make it conductive. For each pixel Pix_(i,j) for which write area WZ_(i,j) has not been altered, the base of via V_(i,j) is electrically insulated from substrate 12. For each pixel Pix_(i,j) for which write area WZ_(i,j) has been altered, everything occurs as if the base of via V_(i,j) were electrically connected to the underlying active area C_(j). As an example, in FIG. 1, pixels Pix_(1,1), Pix_(4,2), and Pix_(3,4) have their base insulated from substrate 12 by insulating layer 18 and the other pixels have their base connected to substrate 12. In FIG. 1, altered write area WZ_(1,2) has been shown by a hatched region.

For each pixel Pix_(i,j) for which the portion of insulating layer 18 separating via V_(i,j) from the underlying active area C_(j) is desired to be altered, the write operation comprises setting the associated metal track R_(i) to a high reference voltage VPP and setting active area C_(j) to a low reference voltage, the other active areas being maintained at voltage VPP. As an example, the low reference voltage corresponds to ground GND and is equal to 0 V and high reference voltage VPP, referenced to ground GND, is greater than some ten volts, for example, on the order of 12 V. High and low reference voltages VPP and GND are selected so that the voltage which settles between via V_(i,j) and the underlying active area C_(j) is sufficiently high to cause the breakdown of the intermediary portion of insulating layer 18 and thus ensure the electric connection between the base of via V_(i,j) and active area C_(j). As an example, insulating layer 18 may correspond to a bilayer having a thickness on the order of 60 nm, comprising an oxide layer on the order of 20 nm covered with a nitride layer on the order of 40 nm. A voltage between via V_(i,j) and active area C_(j) greater than some ten volts enables to ensure the breakdown of write area WZ_(i,j). The breakdowns of the write areas WZ_(i,j) associated with several pixels Pix_(i,j) in the same row may be performed simultaneously. Once the write operation is over, integrated circuit 10 may be used normally.

An image reading operation is performed by removing metal tracks R₁ to R₄ from last metallization level M₄, vias V′_(i,j), and insulating layer INS₄ to expose metal portions Pix_(i,j) of metallization level M₃. By the removal of metal tracks R₁ to R₄, each pixel Pix_(i,j) having an unaltered associated write area WZ_(i,j) is electrically floating. Each pixel Pix_(i,j) for which the associated write area WZ_(i,j) has been altered is electrically connected to the corresponding active area C_(j). According to a variation, only metal tracks R₁ to R₄ are removed. In this case, vias V′_(i,j) play the role of pixels.

A read operation may be performed by passive voltage contrast or PVC imaging methods. Such methods are described, for example, in the work entitled “Microelectronics Failure Analysis: Desk Reference Fifth Edition” published in 2004 by EDFAS Desk Reference Committee. A PVC method comprises projecting an ion or electron beam onto a surface to be observed and detecting the low-energy secondary electrons which are emitted by the surface. An image having its contrast depending on the amount of detected electrons is obtained. In the case of electrically-insulated structures, few secondary electrons are detected. Indeed, positive charges tend to build up at the surface of these structures, thus attracting the secondary emitted electrons which are recaptured. These structures appear as dark on the obtained image. In the case of structures electrically connected to a source of a low reference voltage, for example, the ground, many secondary electrons are detected since there is no electron recapture. These structures appear as lighter.

The PVC method is implemented by setting substrate 12 to low reference voltage GND. This can be done even if integrated circuit 10 is disabled. The setting to the low voltage of substrate 12 enables a carrying off of the charges present in the pixels Pix_(i,j) electrically connected to the corresponding active area C_(j) due to the leakage currents at the junction between active area C_(j) and substrate 12. Pixels Pix_(i,j) thus appear with a light tone on the obtained image while the electrically-isolated pixels Pix_(i,j) appear with a dark tone. Insulating layer INS₃ also appears with a dark tone in the obtained image. In the obtained image, each pixel thus appears according to a two-state luminance attribute: light or dark.

FIG. 3 shows an image obtained by implementation of a method for reading the stored image at the level of circuit 10 of FIG. 1. Pixels Pix_(i,j) are represented by squares. As an example, only pixels Pix_(1,1), Pix_(4,2), and Pix_(3,4), marked with a cross in FIG. 3, are insulated from substrate 12 by insulating layer 18 and appear with a dark tone on the obtained image.

As a variation, the read operation may be implemented by a conduction atomic force microscopy method or C-AFM method. A C-AFM method comprises displacing a tip above the surface of circuit 10, the tip being maintained at a distance from the surface of circuit 10. Substrate 12 is set to a low reference voltages and the tip is set to a high reference voltage. The intensity of the current crossing the tip is measured. The measured intensity varies according to whether the tip is in contact with a floating pixel or with a pixel electrically connected to the substrate. An image is obtained, for example, with a grey level or with a color tone which depends on the intensity of the measured current.

Pixels Pix_(i,j) may be arranged in rows and in columns to form a matrix display. As a variation, pixels Pix_(i,j) may be arranged as segments or diodes of a digital display. As a variation, pixels Pix_(i,j) may have shapes different from one another.

FIGS. 4A to 4D are cross-section views similar to FIG. 2 of structures obtained at successive steps of a method for manufacturing circuit 10 according to an embodiment of the present invention.

FIG. 4A shows the structure obtained after having formed, in substrate 12, active areas C₁ and C₂ separated by insulating regions 16. Insulating regions 16 are, for example, formed by an STI method (shallow trench insulation).

FIG. 4B shows the structure obtained after having successively deposited an oxide layer and a nitride layer forming insulating layer 18.

FIG. 4C shows the structure obtained after having deposited insulating layer INS₁, for example, made of silicon oxide, on insulating layer 18, after having etched openings 22 in layer INS₁, and after having formed vias V_(1,1) and V_(1,2) in openings 22. Insulating layer 18 acts as an etch stop layer in the etching of openings 22. Thereby, vias V_(1,1) and V_(1,2) formed in openings 22 are not in electric contact with the underlying active areas C₁, C₂.

FIG. 4D shows the structure obtained after deposition of insulating layers INS₂, INS₃, and INS₄ and the forming of electric paths EP_(1,1) and EP_(1,2), of pixels Pix_(1,1) and Pix_(1,2), of vias V′_(1,1) and V′_(1,2), and of metal track R₁. An integrated circuit in which all pixels Pix_(i,j) are floating is then obtained. The previously-described write step in which insulating layer 18 is altered is then carried out at the level of certain pixels.

FIG. 5 shows a partial electric diagram of an integrated circuit 30 according to another embodiment of the present invention. In the present embodiment, each pixel Pix_(i,j) is formed by a metal portion of last metallization level M₄. In FIG. 5, two pixels Pix_(1,1) and Pix_(1,2) of a pixel array have been shown. In the present embodiment, for each pixel Pix_(i,j), the associated write area WZ_(i,j) corresponds to the gate insulator of a MOS transistor T_(i,j). Transistor T_(i,j) is a thin-oxide transistor or low-voltage transistor. Further, each metal portion MP_(i,j) is connected by a coupling element CE_(i,j) to a conduction terminal (source or drain) of a power MOS transistor PT_(i,j) having its other conduction terminal connected to a high reference voltage source VPP. Transistor PT_(i,j) is called thick-oxide transistor or high-voltage transistor. Transistor PT_(i,j) for example corresponds to an N-channel MOS transistor. The gate of transistor PT_(i,j) receives a control signal COM_(i,j). Coupling element CE_(i,j) enables, as will be described in further detail hereinafter, to perform the write operation. Advantageously, coupling element CE_(i,j) may correspond to a conventional electronic component such as a memory element of an electrically erasable programmable read-only memory or EEPROM having the structure of a dual-gate MOS transistor having a gate which is floating. Metal portion MP_(i,j) is connected to the floating gate of memory element CE_(i,j) and power transistor PT_(i,j) is connected to the control gate of memory element CE_(i,j). As compared with circuit 10, circuit 30 does not comprise active areas C_(j). Circuit 30 has the advantage of using conventional electronic components.

FIG. 6 is a cross-section view similar to FIG. 2 of pixels Pix_(1,1) and Pix_(1,2), of electric paths EP_(1,1), EP_(1,2), and of coupling elements CE_(1,1) and CE_(1,2) of integrated circuit 30. Each transistor T_(i,j) comprises N-type doped areas 31 extending into substrate 12 on either side of an insulating portion 32 arranged on substrate 12, for example, a thin silicon oxide portion having a thickness of a few nanometers, for example, 2 or 3 nm. A portion 34 of a semiconductor material, for example, polysilicon, covers insulating portion 32. Regions 31 form source and drain regions of transistor T_(i,j). Insulating portion 32 forms the gate insulator of transistor T_(i,j) and corresponds to write area WZ_(i,j). Semiconductor portion 34 forms the gate of transistor T_(i,j). An insulating layer, not shown, may cover pixels Pix_(i,j).

Each memory element CE_(i,j) comprises N-type doped regions 35 extending into substrate 12 on either side of an insulating portion 36 arranged on substrate 12. Insulating portion 36 comprises a thick region 38, for example having a thickness on the order of 20 nm and a thinner region 40 for example having a thickness on the order of some ten nanometers. A conductive portion 42, for example, made of polysilicon, covers insulating portion 36. Portion 42 forms the floating gate of memory element CE_(i,j). An insulating portion 44 partially covers semiconductor portion 42. A semiconductor portion 46 covers insulating portion 44. Portion 46 forms the control gate of memory element CE_(i,j).

With respect to circuit 10, for each pixel Pix_(i,j) of circuit 30, via V_(i,j) connected to gate 32 of transistor T_(i,j). Metal track MP_(i,j) is connected to via V_(i,j) and, further, to a via 48 having its base connected to floating gate 42 of memory element CE_(i,j). A metal track 50 of the first metallization level connected, by a via 52 crossing insulating layer INS₁, to control gate 46 of memory element CE_(i,j) has also been shown in FIG. 6, associated with each pixel Pix_(i,j).

The writing of an image into integrated circuit 30 is carried out by altering the gate insulator 32 of transistor T_(i,j) of certain pixels Pix_(i,j) so that via V_(i,j) pixel Pix_(i,j) is electrically connected to underlying substrate 12. In FIG. 6, gate insulator 32 of transistor T_(1,2) has been shown in hatching to show that it has been altered.

The writing step is carried out by setting substrate 12 to low reference voltage GND and by turning on the transistor PT_(i,j) of each pixel Pix_(i,j) which is desired to be electrically connected to substrate 12. The voltage of control gate 46 of memory element CE_(i,j) then tends to substantially rise up to high reference voltage VPP. The stack of conductive and insulating portions 36, 42, 44, and 46 of memory element CE_(i,j) forms a capacitive bridge. This results in a rise in the voltage of floating gate 42 of memory element CE_(i,j) and thus in a rise in the voltage of gate 34 of transistor T_(i,j) to a value sufficient to cause the breakdown of gate insulator 32 of transistor T_(i,j). For example, a voltage between via V_(i,j) and substrate 12 greater than some ten volts is sufficient to obtain the breakdown of a silicon oxide gate insulator 32 having a thickness of a few nanometers. To improve the rise of the voltage of gate 34 of transistor T_(i,j), source and drain regions 35 of memory element CE_(i,j) may be brought up to VPP during the write operation.

After the write operation, certain pixels Pix_(i,j) are thus electrically isolated from substrate 12 by transistor T_(i,j) having its gate insulator 32 left intact and by memory element CE_(i,j) having a floating gate 42. Other pixels Pix_(k,l) are electrically connected to substrate 12 via transistor T_(k,l) having an altered gate insulator 32.

The read step is carried out as described previously with circuit 10 of FIGS. 1 and 2, but for the difference that only the metal portions of the last metallization level need to be exposed. Indeed, a PVC or C-AFM method may be carried out directly at the level of the metal portions of the last metallization level.

According to a variation of circuit 30, transistors T_(i,j) may be replaced with a structure similar to circuit 10 in which each via V_(i,j) is initially insulated from substrate 12 by an insulating layer 18 which is altered or left as such during the write operation.

FIGS. 7A to 7F are drawings similar to FIG. 6 and show structures obtained at successive steps of a method for manufacturing circuit 30 according to an embodiment of the present invention.

FIG. 7A shows the structure obtained after the steps of:

forming, in substrate 12, separation regions 16 and N-type doped regions 31, 35;

depositing over the entire circuit an insulating layer 54, for example, a silicon oxide layer having a thickness on the order of 20 nanometers; and

forming in oxide layer 54 thinned-down portions 56 at the locations of the memory elements. Each thinned-down portion 56 has, for example, a thickness smaller than 10 nanometers.

FIG. 7B shows the structure obtained after having deposited, over the entire circuit, a polysilicon layer having, for example, a 100-nanometer thickness, after having etched the polysilicon layer to delimit, at the level of the locations of each memory element, a polysilicon portion 58, and after having covered the entire circuit with an insulating layer 60 having a thickness on the order of 20 nanometers, corresponding, for example, to a silicon oxide layer or to a multiple-layer structure comprising a stacking of a silicon oxide layer, a silicon nitride layer, and a silicon oxide layer.

FIG. 7C shows the structure obtained after having etched insulating layers 54, 60 to delimit, at the locations of the memory elements, a portion 62 of insulating layer 54 and a portion 64 of insulating layer 60 on either side of polysilicon portion 58, insulating layers 54, 60 being removed at the locations of the transistors.

FIG. 7D shows the structure obtained after having formed an insulating layer 66, for example, a silicon oxide layer having a thickness of a few nanometers, on the circuit portions not covered with insulating layers 64 and after having covered the entire circuit with a polysilicon layer 68 for example having a thickness from 100 to 200 nanometers.

FIG. 7E shows the structure obtained after having carried out the steps of etching polysilicon layer 68, insulating portions 64, polysilicon portions 58, insulating portions 62, and insulating layer 66, to delimit, for each memory element CE_(i,j), insulating portion 36, floating gate 42, insulating portion 44, and control gate 46, and to delimit for each transistor T_(i,j), gate insulator 32 and gate 34.

FIG. 7F shows the structure obtained after having covered the previous structure with the stacking of insulating layers INS₁, INS₂, INS₃, and INS₄ while forming the associated vias and metal portions. At the end of the previously-described method, each pixel Pix_(i,j) is floating. The method carries on with the writing step.

In the previously-described embodiment, memory element CE_(i,j) corresponds to a conventional memory element of an EEPROM. In this case, memory element CE_(i,j) is associated with a selection transistor, not shown, and may be used, after a write operation, to verify whether the write operation has taken place properly. Indeed, for a pixel Pix_(i,j) for which gate insulator 32 of transistor T_(i,j) has not been altered, it is no longer possible to store charges in floating gate 42 of memory element CE_(i,j) since these charges will be carried off towards substrate 12 by transistor T_(i,j). The absence of charges in floating gate 42 of memory element CE_(i,j) may be detected by a read operation of memory element CE_(i,j).

One advantage of the previously-described embodiment is that the writing step is simplified compared with circuit 10. Indeed, during the writing step, a unique reference voltage is applied to the substrate, and each pixel can be addressed individually thanks to its associated selection element PTi,j. This allows, in particular, carrying out parallel writing operations on pixels from different lines and columns.

Another advantage of the previously described embodiment is that the reading step is simplified compared with circuit 10. Indeed, each pixel corresponds to a metal portion of the ultimate metal level M4. Thus, pixels are directly reachable for reading, without a previous step of removal of the metal tracks.

According to an alternative embodiment, insulating portion 38 of memory element CE_(i,j) comprises no thinned-down area. Further, memory element CE_(i,j) comprises no source and drain regions 35 and transistor T_(i,j) comprises no source and drain regions 31.

FIG. 8 shows a partial electric diagram of an integrated circuit 70 according to another embodiment of the present invention. In the present embodiment, pixels Pix_(i,j) correspond to metal portions of penultimate level M₃. In FIG. 8, two pixels Pix_(1,1) and Pix_(1,2) of a pixel array have been shown. The electric diagram of circuit 70 is similar to the electric diagram of circuit 30 of FIG. 5, with the difference that coupling element CE_(i,j) is replaced with a conduction path S_(i,j) electrically connecting metal portion MP_(i,j) to a conduction terminal of MOS power transistor PT_(i,j), the other conduction terminal of transistor PT_(i,j) being connected to the source of high reference voltage VPP. Transistor PT_(i,j) corresponds, for example, to a P-channel MOS transistor. Connection path S_(i,j) corresponds to the serializing of vias and of metal portions of different metallization levels. In particular, conduction path S_(i,j) comprises a metal portion of the penultimate level. The method for manufacturing circuit 70 uses certain steps of the method for manufacturing circuit 30, connection path S_(i,j) being formed in parallel with electric path EP_(i,j).

In the same way as for circuit 30 of FIG. 5, a write operation comprises altering, for certain pixels Pix_(i,j), the gate insulator of the associated transistor T_(i,j). For this purpose, the associated power transistor PT_(i,j) is turned on and substrate 12 is set to the low reference voltage. This results in a rise of the gate voltage of transistor T_(i,j). The voltage obtained between the gate of transistor T_(i,j) and the underlying substrate is sufficiently high to cause the breakdown of transistor T_(i,j). Transistor PT_(i,j) is a power transistor capable of accepting voltage VPP.

In the same way as for circuit 10 shown in FIG. 2, a read operation of circuit 70 is performed by removing the metal portions from the last metallization level. An interruption of conduction path S_(i,j) is thus obtained. Thereby, each pixel Pix_(i,j) having an unaltered write area W_(i,j) is floating while each pixel Pix_(k,l) having an altered write area W_(k,l) is electrically connected to substrate 12. Substrate 12 is then set to low reference voltage GND and a method of PVC or C-AFM type may be implemented as described previously.

FIG. 9 is a cross-section view of pixel Pix_(1,1), of conduction path S_(1,1), and of transistor PT_(1,1). Transistor PT_(1,1) is, as an example, a P-type MOS transistor formed at the level of an N-type well 72 which extends into substrate 12. Transistor PT_(1,1) comprises doped regions 74, 76 which extend in well 72 on either side of an insulating portion 78 covering substrate 12. A conductive portion 80 covers insulating portion 78. Region 74 corresponds to the drain of transistor PT_(1,1) and region 76 corresponds to the source of transistor PT_(1,1). Region 76 is connected to the source of voltage VPP, not shown in FIG. 9. As an example, conduction path S_(1,1) comprises a metal portion 82 of metallization level M₄ which is connected, at one end, to metal portion MP_(1,1) by a first stacking 84 of vias and of metal portions of metallization levels M₂ and M₃ and is connected, at the other end, to drain 74 of transistor PT_(1,1) by a second stacking 86 of vias and of metal portions of metallization levels M₁ to M₃. A metal portion 88 of the first metallization level connected to gate 80 of transistor PT_(1,1) by a via 90 has further been shown in FIG. 9.

FIG. 10 shows a partial electric diagram of an integrated circuit 100 according to another embodiment of the present invention. In the present embodiment, pixels Pix_(i,j) correspond to metal portion of last metallization level M₄. In FIG. 10, a pixel Pix_(1,1) of a pixel array has been shown. Each pixel Pix_(i,j) has a structure similar to that of circuit 30. However, for circuit 100, coupling element CE_(i,j) corresponds to a MOS capacitor. It for example is a so-called thick oxide MOS transistor having its conduction terminals (source and drain) connected to power transistor PT_(i,j) and having its gate connected to pixel Pix_(i,j). The method for manufacturing circuit 100 uses some steps of the method for manufacturing circuit 30.

In the same way as for circuit 30 of FIG. 5, an image writing operation in circuit 100 comprises altering, for some pixels Pix_(i j), the gate insulator of the associated transistor T_(i,j). For this purpose, the associated power transistor PT_(i,j) is turned on and substrate 12 of circuit 100 is set to low reference voltage GND. This results in a rise of the gate voltage of transistor CE_(i,j) and, by capacitive coupling, in a rise of the voltage at the source and drain of this transistor. This translates as a sufficient rise of the gate voltage of transistor T_(i,j). The voltage between the gate of transistor T_(i,j) and the underlying substrate 12 is sufficiently high to cause the breakdown of the gate insulator of transistor T_(i,j).

An read operation of the image stored in circuit 100 is performed in the same way as for circuit 30 of FIG. 6, with thick-oxide transistor CE_(i,j) ensuring the electric insulation of pixel Pix_(i,j).

FIG. 11 is a cross-section view of pixel Pix_(1,1), of the associated electric path EP_(1,1) and coupling element CE_(1,1). Coupling element CE_(1,1) is, as an example, a P-type MOS transistor formed at the level of an N-type well 102 which extends in substrate 12. Transistor CE_(1,1) comprises doped regions 104 which extend in well 102 on either side of an insulating portion 106 covering substrate 12. A conductive portion 108 covers insulating portion 106. Regions 104 correspond to the drain and to the source of transistor PT_(1,1). Regions 104 are connected to a conduction terminal of power transistor PT_(1,1), not shown. Insulating portion 106 forms the gate insulator of transistor CE_(1,1) and conductive portion 108 forms the gate of transistor CE_(1,1). The metal portion MP_(1,1) associated with pixel Pix_(1,1) is connected by a via 110 to gate 108 of transistor CE_(1,1).

Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, although in the previously-described embodiments, the integrated circuits comprise four metallization levels, it should be clear that the present invention may be implemented for an integrated circuit comprising a different number of metallization levels.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 

What is claimed is:
 1. A method for storing an image in an integrated circuit, comprising: forming an array of pixels on a substrate, each pixel comprising a metal portion of a metallization level or a via level and a programmable insulating portion connected between the metal portion and the substrate; and programming the array of pixels by altering the insulating portion of selected pixels to cause the metal portion to be electrically connected to the substrate, wherein the selected pixels have a first state and non-selected pixels have a second state, and wherein the states of the pixels in the array of pixels define an image stored in the integrated circuit.
 2. A method for storing an image as defined in claim 1, further comprising forming metal tracks of a last metallization level configured to address selected pixels of the array of pixels during programming thereof.
 3. A method for storing an image as defined in claim 1, wherein forming an array of pixels further comprises, for each pixel, forming a dual gate transistor having a first gate connected to the metal portion and a second gate connected to a selection element configured to program the pixel.
 4. A method for storing an image as defined in claim 1, wherein forming an array of pixels further comprises, for each pixel, forming a MOS capacitor having a first terminal connected to the metal portion and a second terminal connected to a selection element configured to program the pixel.
 5. A method for storing an image as defined in claim 1, wherein forming an array of pixels further comprises, for each pixel, forming a capacitive coupling element having a first terminal connected to the metal portion and a second terminal connected to a selection element configured to program the pixel.
 6. A method for storing an image as defined in claim 1, wherein forming an array of pixels further comprises, for each pixel, forming a removable conduction path connected between the metal portion and a selection element configured to program the pixel.
 7. A method for storing an image as defined in claim 1, further comprising optically reading the electrical state of the pixels in the array of pixels.
 8. A method for storing an image as defined in claim 1, further comprising reading the electrical states of the pixels in the array of pixels by passive voltage contrast imaging.
 9. A method for storing an image as defined in claim 1, further comprising reading the electrical states of the pixels in the array of pixels by conduction atomic force microscopy. 